1. Field of the Invention
The invention is in the field of concrete compositions, particularly concrete having decreased rate of water vapor emission, lower internal relative humidity, and faster surface drying after hardening.
2. Related Technology
Concrete is a composite construction material composed primarily of the reaction products of hydraulic cement, aggregates, and water. Water is both a reactant for the cement component and is necessary to provide desired flow characteristics (e.g., spread and/or slump) and ensure consolidation of freshly mixed concrete to prevent formation of strength-reducing voids and other defects. Chemical admixtures may be added to freshly mixed concrete to modify characteristics such as rheology (i.e., plastic viscosity and yield stress), water retention, and set time. Although some of the water reacts with the cement component to form crystalline hydration products, a substantial portion remains unreacted and is typically gradually removed from concrete by evaporation. The continuing presence of water in concrete can pose problems, particularly when applying a floor covering.
The discontinued use of volatile components in floor covering adhesives for concrete surfaces has created bonding and delamination problems. Concrete contains reaction water for cement hydration and water of convenience to facilitate workability and placement. Water is both chemically bound and entrapped in gel and small capillaries comprising about 30-50% of the paste material, depending upon maturity. Water in concrete must be consumed, sequestered, or evaporated into the atmosphere before a proper, permanent water-based adhesive bond can be assured. Unfortunately, the time necessary to accommodate the requisite drying process is approximately one month per inch of concrete floor depth for standard weight concrete.
A cementitious composition for forming concrete generally refers to a mixture of natural and/or artificial aggregates, such as, for example, sand and either gravel or crushed stone, which are held together by cement paste binder to form a highly durable building material. Cement paste is typically made up of the reaction products of hydraulic cement, such as Portland cement, and water. Cement paste may also contain one or more chemical admixtures as well as supplementary cementing materials, such as, for example, fly ash or ground granulated blast furnace slag cement (GGBFS).
Blended cements can comprise one or more pozzolan materials, which are primarily glassy or non-crystalline siliceous or aluminosiliceous materials that are hydraulically reactive and have cementitious properties in the presence of excess calcium hydroxide provided by hydrating Portland cement. The silicates and aluminates of a pozzolan reacting with excess calcium hydroxide form secondary cementitious phases (e.g., calcium silicate hydrates similar to those from Portland cement, but possibly having a lower calcium to silicate ratio), which provide additional strengthening properties that usually begin after about 7 days of curing.
Blended hydraulic cement may comprise up to 40% or more fly ash, which reduces the amount of water that must be blended with the cementitious composition, allowing for improvement in later strength as the concrete cures. Other examples of pozzolans that can be used in hydraulic cement blends include highly reactive pozzolans, such as silica fume and metakaolin, which further increases the rate at which the concrete gains strength, resulting in a higher early strength. Current practice permits up to 40% or more reduction in the amount of hydraulic cement used in the concrete mix when replaced with a combination of pozzolans that do not significantly reduce the final compressive strength or other performance characteristics of the resulting concrete.
Lightweight aggregates are frequently designed into a concrete mix to reduce building dead load, enable longer spans, provide better seismic benefits, increase fire resistance, and improve sound insulation. This lightweight material commonly comprises expanded shale, clay, pumice, cinders, or polystyrene with a density of about ½ or less than that of normal aggregates and is capable of producing concrete that weighs from 800 to 1000 pounds less per cubic yard.
In general, the weight reduction in lightweight aggregate is achieved by creating a highly porous internal structure that can, unfortunately, also absorb up to 30% water. This water is in addition to the normal water of convenience and can impart an additional amount to the concrete mix equal to 2-3 times of that which must normally be consumed and evaporated, thereby further increasing the time-to-dry for adhesive or epoxy application. To prevent workability losses due to water absorption during mixing, transport and placement, porous aggregates are often pre-conditioned with water.
Should the concrete be conveyed to the location of placement by a concrete pump, water absorption by the porous aggregates becomes more critical, since the concrete may be subjected to liquid pressure within the pump and attendant line of up to 1000 psi (69 bar), which greatly compresses the air in the pores and causes significant additional water absorption. Such pressure can force water required for workability into previously unsaturated pores of lightweight aggregates (i.e., pores which are not filled when subjected to atmospheric pressure but which can be filled at high pressures associated with pumping). Thus, complete saturation of the pores of lightweight aggregates is preferred to prevent workability loss and potential pump line obstructions under these conditions.
Unfortunately, complete saturation is impractical since prolonged soaking in water will not displace air trapped within the capillaries of the lightweight aggregate, so some loss of mix water during conveyance has to be tolerated. Moreover, water instilled into porous aggregates may quickly evaporate in storage, returning the lightweight aggregate largely to its previous dry condition within days. Thus, pre-wetted aggregates must be used almost immediately to capture the desired benefit.
Moreover, even these methods often do not typically result in fully saturated capillaries. Any remaining empty capillaries, when subjected to pump pressures, partially fill with water in response, compressing the air trapped in the capillaries of the lightweight in accord with the Universal Gas Law, thus resulting in the aforementioned workability losses and potential line clogging during pumping. This can have several consequences: additional water must be added to the concrete mix prior to pumping to maintain workability sufficient to facilitate pumping. Thereafter, when the concrete exits the pump and returns to normal atmospheric pressure, the excess water responds to the compressed air within the lightweight aggregates and is partially forced back out into the mix. This, in effect, increases the water-to-cement ratio, excessively diluting the plastic concrete mix and impacting the hardened concrete's permeability.
The cementitious materials in concrete require water, typically known as chemical water or hydration water, to chemically evolve into a hard, crystalline binder. For example, Portland cements generally require up to about 40% of their weight in water in order to promote complete hydration and chemical reaction.
Excess water has conventionally been added to make concrete more plastic allowing it to flow into place. This excess water is known as water of convenience. A small amount of the water does escape as a result of solids settling during the plastic phase, evaporation at the atmospheric interface, and absorption into accepting interface materials. However, much of the water of convenience remains in the concrete during and immediately following hardening. The water of convenience can then escape into the atmosphere following the hardening of the concrete. The water of convenience, depending on, among other things, the water to cementitious ratio, may represent up to about 70% of the total water in the concrete.
The concrete construction and floor-covering industries may incur both construction delays and remedial costs as a result of water vapor emissions and water intrusion from concrete. For example, adhesives and coatings used in the construction of concrete floors are relatively incompatible with moisture that develops at the concrete surface. Moisture may also create an environment for promoting the growth of mold.
Water tightness in concrete structures is a measure of the ability of the hardened concrete to resist the passage of water. Water vapor emission is proportional to the state of relative dryness of the body of the concrete structure. Once isolated from external sources of water, water vapor emissions are derived from the amount of water that is used in excess of that needed to harden the cementitious materials—i.e., the water of convenience. Depending upon the atmospheric temperature and humidity at the surface and the thickness of the concrete, the elimination of excess water through water vapor emissions can take months to reach a level that is compatible with the application of a coating or an adhesive.
There is also a possibility that water may develop under the floor due to flooding, water backup, etc. Hardened concrete that resists water vapor permeation is capable of further protecting any coatings that have been applied to the surface of the concrete. There is a need in the art for a concrete that, once it becomes hardened, is substantially resistant to water vapor permeation.
Installation of an impermeable barrier on the surface of the concrete prior to reaching an acceptable level of dryness may result in moisture accumulation, adhesive failure, and a consequential failure of the barrier due to delamination. Premature application of coatings and adhesives increases the risk of failure, while the delay caused by waiting for the concrete to reach an acceptable level of dryness may result in potentially costly and unacceptable construction delays.
The floor covering industry has determined, depending on the type of adhesive or coating used, that a maximum water vapor emission rate of from 3 to 5 pounds of water vapor per 1,000 square feet per 24 hour period (lb/1000 ft2/24 h) is representative of a state of slab dryness necessary before adhesive may be applied to the concrete floor. Accordingly, there remains a long-felt but unsatisfied need in the art for cementitious compositions that reduce the amount of time needed to reach a desired water vapor emission rate in concrete floors enabling a more timely application of coatings and adhesives.
It is known in the art that certain polymers classified as superplasticizers may be included in concrete in order to reduce the amount of water of convenience needed to allow the cementitious mix to more readily flow into place. Certainly, a reduction in the amount of excess water remaining after the concrete hardens should lead to a reduction in the amount of time necessary to reach a desired water vapor emissions rate. However, the use of superplasticizers alone does not address other effects that influence the rate of water vapor emission from the concrete.
Accordingly, there remains a need in the art for cementitious compositions that further reduce the amount of time necessary to reach a desired water vapor emission rate in concrete floors beyond that which is achieved through a reduction in the amount of water required through the use of a superplasticizer additive.
If attainment of faster drying lightweight concrete is an objective, the usual method of water reduction by utilizing large doses of superplasticizers (very high range water reducers) is difficult because of the sensitivity of the mix to the loss of the enhanced efficiency water (Field workability consistency). Furthermore, high doses of superplasticizers tend to impart a thixotropic characteristic exhibited by workability loss if deprived of mixing shear. This loss of mixing shear often occurs during pump hose movement or delay in concrete supply. Because the efficiency of admixture-treated water is improved, loss of water by temporary absorption into the pores of lightweight aggregates during pressurized pumping has both a substantially greater negative impact on workability and a greater negative impact causing potential segregation and bleeding when the admixture-treated water is released from the pores of the aggregates after exiting the pump.
Similarly, the inclusion of silica fume or metakaolin both well-known. Highly reactive pozzolans possess very high surface areas and therefore again require super-plasticizer to reduce water and maintain workability. It also has been found that highly super-plasticized concrete is more difficult to air entrain. Air entrainment is an important feature of lightweight concrete, since it aids in reducing weight and lowers the mortar density thereby attenuating the tendency of the coarse lightweight aggregate particles to float to the surface and hinder finishing operations.
The absorbed water and resulting added mixture water caused by pumping concrete containing porous lightweight aggregates therefore poses difficulties when accelerated drying of the concrete is desired. As a consequence of concrete hydration and lowering of internal vapor pressure in the mortar, the additional water released from the capillaries of the porous aggregates permeates the mortar in the concrete. While this can be beneficial from the standpoint of promoting more complete hydration of the cementitious binder, particularly in lower water-to-cement ratio systems, it can create a prolonged period of relatively high humidity within the concrete, resulting in moist concrete that must dry out before it can be coated or sealed. Such drying is further retarded in humid climates.
The state of dryness within concrete is usually determined by drilling holes to accommodate in situ humidity probes. When these probes are installed at the depth required by ASTM F-2170-09, it is presumed to be representative of the future sealed equilibrium moisture condition of the full concrete thickness. Attainment of 75-80% relative humidity (some floor coverings tolerances may be slightly more or less) ensures that the concrete surface is ready for adhesive application. Experience in the floor covering industry has validated research data which indicates that if internal humidity probes are inserted to a depth of 40% of a concrete structure having one side exposed to the atmosphere (20% if two sides are exposed) in accordance with ASTM F-2170-09, “Standard Method for Determining Humidity in Concrete Floor Slabs Using in situ Probes”, and the probes indicate an internal relative humidity of 75-80%, that this is representative of the sealed future equilibrium moisture condition of the full thickness. If the internal relative humidity is higher than 75-80%, it is assumed the floor will not accept water based glue and will generate sufficient vapor pressure to delaminate impervious coatings. Below that amount, and absent outside moisture influences, it is assumed the structure can accept water based glue and not generate sufficient vapor pressure differential to de-bond impervious coatings. Epoxy sealers are also sensitive to water vapor pressure and consequently, encounter similar problems. Premature application of either water-soluble adhesive or epoxy sealer to under-dried concrete can result in moisture accumulation beneath the applied impervious surface and a potential for loss of bond with the epoxy or flooring. There are sealers that can be applied to attenuate the water vapor emission, but they often fail, resulting in loss of space utilization during repair and occasionally creating costly litigation. To reduce the risk of such problems, floors with excessive humidity may require drying times of up to a year or more.
The substitution of the porous lightweight aggregates which absorb water instead of normal aggregates can prolong drying times by months or a year or more. Research has demonstrated that high performance standard weight coarse aggregates concrete (HPC) can dry to satisfactory IRH condition comparatively rapidly. These concretes have a water-to-cementitious ratio (w/cm) generally below 0.40 and can contain fairly large amounts of cement or cement/pozzolans to achieve an internal relative humidity of 75-80% as determined by ASTM F 2170. An example of the large water difference is shown in Table 1 below.
TABLE 1dry, lbsHighdry, lbsdry, lbsPerformanceLightweightNormalConcrete (HPC)HPCCement300400400GGBFS200400400Sand134012741220Stone17501750Lightweight850Water325285325plasticizer10 oz40 oz.40 oz.W/C0.650.360.41PCF145150.5118.3AE1.30%1.30%5%Total W/C0.700.390.60Aggregate2323151Water
Other research by Suprenant and Malisch (1998) reported that a 4 inch concrete slab made from conventional concrete required 46 days to reach a moisture vapor emission rate (MVER) of 3.0 lb/1000 ft2/24 h. In 1990 they reported that a lightweight concrete slab made with the same w/cm and cured in the same manner took 183 days to reach the same MVER, a four foldincrease.
The construction industry, therefore, faces a dichotomy. It can address water absorption by the porous aggregate with as much water as needed to ensure pumpability and avoid critical workability loss in the pump line and deal with the consequent prolonged drying time of up to a year or accept the risk of floor failure by using a sealer to isolate the moisture-laden floor from an applied impervious coating or water soluble glue. The concrete construction and floor-covering industries may therefore incur construction delays and/or remedial costs as a result of water vapor emissions and water intrusion from concrete. Moisture may also create an environment for promoting growth of mold.